The Relevant Technology
Self-aligned structures in an integrated circuit may be formed by shielding previously formed raised structures with a protective layer. An etchable layer is then typically deposited over the shielded previously formed structures, and a masking layer is deposited on the etchable layer and patterned. The etchable layer is then etched. The protective layer resists etching and shields the previously formed structures, effectively steering the etch around and between the previously formed structures, leaving spaces free of the etchable layer which are self-aligned to the previously formed structures. A fill material is then deposited to fill the spaces, forming structures comprised of fill material self-aligned to the previously formed structures.
The material which must be etched to form contact structures in a MOS process is typically an oxide of silicon such as borophosphorous silica glass (BPSG). Silicon nitride or silicon-rich silicon oxide is typically used to form protective layers resistant to silicon oxide etching. Silicon nitride is typically preferred. Protective layers are formed over structures such as wordlines around which the contacts must be routed. Etchants selective to the material of the protective layer etch the BPSG preferentially to the protective layer, resulting in spaces which are self-aligned to the surrounding structures. These spaces are then filled with a conductive material or materials to form the self-aligned contacts.
The higher the selectivity of the BPSG etchant to the material of the protective layer, the less likely the protective layer is to be breached during etching of the contact spaces. Increased etch selectivity can thus improve the reliability, control, and yield of the process of forming self-aligned contacts, potentially allowing denser spacing of circuit devices and improving process yields for the contact forming process.